1. Field of the Invention
The present invention relates to a process for the production of reinforced slabs of stone material and also to the reinforced slabs of stone material so obtained.
More specifically, the present invention relates to a process for the production of slabs of stone material which have a small thickness and which exhibit a high level of resistance to both positive and negative flexing. These slabs are usually used as coverings for masonry structures, for example for pavings, for facing walls of buildings and, more generally, for applications in which the mechanical characteristics must be accompanied by special aesthetic effects. It is well known that, in the case of natural stone materials in the form of slabs, the main objective has always been, and still is, to produce slabs that are as thin as possible (for obvious reasons) without the mechanical properties being diminished to an unacceptable extent. With regard to thin slabs, a problem of not inconsiderable importance from the point of view of use is that of the manipulation of the slabs which have to be handled with great care to avoid breakages or cracks. Therefore, both during transport and in the course of their manipulation, for example during the moving operations which precede final installation, it is necessary to avoid any situation in which the slab is supported in a cantilevered manner, because any oscillation, however slight, of the free edge of the slab will inevitably lead to breakage.
2. Description of the Prior Art
European Patent Application No. 95201590.7, filed Jun. 15, 1995 and U.S. patent application Ser. No. 08/513,687, filed Aug. 11, 1995, now U.S. Pat. No. 5,670,007 in the name of Marcello Toncelli, the inventor of the subject matter of this application, describes a process for the production of thin slabs of stone material, such as marble and granite, which are reinforced at their rear face, or in other words the face that is not visible, which process basically consists in applying to the above-mentioned rear, or invisible, face of the slab a layer of hardenable resin, for example an epoxy resin, while incorporating in the resin layer a reinforcing material comprising non-twisted linear elements of glass threads, preferably in the form of a mat, and maintaining a specific percentage ratio by weight between the resin and the mat of glass threads. According to one variant, it is possible to form, in the rear face of the slab, cavities or grooves which accommodate additional linear reinforcing elements, preferably in the form of glass fibre strands, which are thus also embedded in the resin layer. U.S. patent application Ser. No. 08/513,687 corresponds to the above-mentioned European Patent Application No. 95201590.7 filed Jun. 15, 1995 in the. European Patent Office.
My earlier invention is concerned with a process for producing reinforced slabs of products made of stone material, having a reinforcement which includes a hardened resin combined with a rear face of the slab, and includes the provision of a slab stone material having a rear substantially smooth face free of grooves or recess, and in which non-twisted linear reinforcing elements are provided, and the non-twisted linear reinforcing elements are coated with a coating of a resin to form a reinforcement having a percentage ratio by weight of resin to the non-twisted linear reinforcing elements of at most 50:50, and a reinforcing layer is inserted between the coated non-twisted reinforcing elements and the rear face of the slab of stone material, and the resin is hardened.
Further features of my earlier invention are that the glass strands are in the form of a matting, the percentage weight ratio of the resin to the glass-strand matting is 33:66. In one embodiment, the linear reinforcing elements are made of metal, such as steel, and the linear reinforcing elements are housed in grooves or recesses formed on the rear face of the slab. Other features of my earlier invention with respect to the non-visible face of the slab of stone material are incorporated herein by reference.
With respect to the hardening of the resin, this is performed by either adding a catalyst to the resin, and/or the application of heat to the resin, or a combination of the addition of a catalyst and heat. A further feature of my earlier invention includes the application of two mats of non twisted glass strands impregnated with 33% by weight of an epoxy resin, to provide for a linear expansion coefficient of the mat and resin combination between 15 and 30.times.10.sup.-6 per .degree.C.
For further details on the method of implementing this process, the technical problem which this process solves and the properties of the resulting reinforced slabs, reference should be made to the text of the above-mentioned U.S. patent application which is incorporated herein by reference.
In particular, it is noteworthy that the flexural resistance of the slabs of stone material thus reinforced is greatly increased. For example, in the case of a granite slab from 10 to 12 mm thick, of which the flexural resistance is normally of the order of 150 kg/cm.sup.2 by producing the reinforcement using two mats of glass threads and epoxy resin, it is found that the resulting composite slab, always for a total thickness of 10 mm, exhibits a flexural resistance of approximately 1350 kg/cm.sup.2 In the case of facings for the external walls of buildings, it is, however, necessary also to bear in mind another resistance problem which is associated specifically with the action of atmospheric agents and, in particular, winds.
The flexural resistance is increased by the process disclosed in the above-mentioned U.S. patent application in respect of the reinforced face, while, for the face which is in view, this resistance remains at the usual values of from 120 to 180 kg/cm.sup.2, depending upon whether the material concerned is marble, granite or another natural stone material.
When the building is subjected to winds of substantial force and speed, a partial vacuum, which can also cause the breakage of some of the slabs, with the obvious disadvantages, acts on the slabs covering the face of the building that is not directly subjected to the wind current, especially in the region of the vertical edges of the building.
On the other hand, no means have hitherto been known for increasing the flexural resistance of these slabs in respect of the face that is to form the visible face, for which there are absolutely essential requirements of a formal and aesthetic nature.
Therefore, the only solution for facing buildings situated in regions subjected to strong winds has hitherto been to adopt slabs of stone material of substantial thickness in order to compensate for the lesser flexural resistance.